Creates the lobby for a claimed match and notifies the players.

Runs outside the sweep's cluster lock: Lobbies.create_lobby/1 and join_lobby/2 fire plugin hooks and broadcasts, which must never run inside a transaction. The tickets are already claimed (status matched), so no other node can pick them up meanwhile; on failure they are requeued and retried on the next tick.

Errors return tuples instead of raising, so one bad match never aborts the rest of the sweep.